The only realistic alternative would be to launch air strikes at Iran's nuclear sites, a potential disaster. Iran has more than 70 such sites, many of them in urban areas, and some of them buried underground. So an attack would not be surgical or quick. Military officials say it would take weeks of repeated poundings, with unavoidable civilian deaths. A bipartisan group of senior American military and diplomatic officials studied the consequences and warned that it would set back Iran's nuclear program by only a few years, while hardening its resolve to build weapons.